MinC/MinD copolymers are not required for Min function

In Escherichia coli, precise placement of the cytokinetic Z ring at midcell requires the concerted action of the three Min proteins. MinD activates MinC, an inhibitor of FtsZ, at least in part, by recruiting it to the membrane and targeting it to the Z ring, while MinE stimulates the MinD ATPase inducing an oscillation that directs MinC/MinD activity away from midcell. Recently, MinC and MinD were shown to form copolymers of alternating dimers of MinC and MinD, and it was suggested that these copolymers are the active form of MinC/MinD. Here, we use MinD mutants defective in binding MinC to generate heterodimers with wild‐type MinD that are unable to form MinC/MinD copolymers. Similarly, MinC mutants defective in binding to MinD were used to generate heterodimers with wild‐type MinC that are unable to form copolymers. Such heterodimers are active and in the case of MinC were shown to mediate spatial regulation of the Z ring demonstrating that MinC/MinD copolymer formation is not required. Our results are consistent with a model in which a membrane anchored MinC/MinD complex is targeted to the Z ring through the conserved carboxy tail of FtsZ leading to breakage of FtsZ filaments.     

Determination of the structure of the MinD-ATP complex reveals the orientation of MinD on the membrane and the relative location of the binding sites for MinE and MinC

The three Min proteins spatially regulate Z ring positioning in Escherichia coli and are dynamically associated with the membrane. MinD binds to vesicles in the presence of ATP and can recruit MinC or MinE. Biochemical and genetic evidence indicate the binding sites for these two proteins on MinD overlap. Here we solved the structure of a hydrolytic-deficient mutant of MinD truncated for the C-terminal amphipathic helix involved in binding to the membrane. The structure solved in the presence of ATP is a dimer and reveals the face of MinD abutting the membrane. Using a combination of random and extensive site-directed mutagenesis additional residues important for MinE and MinC binding were identified. The location of these residues on the MinD structure confirms that the binding sites overlap and reveals that the binding sites are at the dimer interface and exposed to the cytosol. The location of the binding sites at the dimer interface offers a simple explanation for the ATP dependence of MinC and MinE binding to MinD.     

A conserved sequence at the C-terminus of MinD is required for binding to the membrane and targeting MinC to the septum

MinD is a key component of an oscillatory system that spatially regulates cell division in Escherichia coli. It is a peripheral membrane ATPase that recruits MinC and oscillates between the two halves of the cell in a MinE dependent manner. In vitro MinD binds to phospholipid vesicles in an ATP-dependent manner and is released through MinE-stimulated ATP hydrolysis. In this study we examined the function of the conserved C-terminus of MinD. Short truncations of three and ten amino acids dramatically decreased the ability of MinD to localize to the membrane and spatially regulate division. These truncations bound MinC but were deficient in targeting MinC to the septum. In vitro they dimerized, but were deficient in binding to phospholipid vesicles and undergoing MinE stimulation. We suggest a model in which the ATP-dependent dimerization of MinD affects the conformation of the C-terminal region, a potential amphipathic helix, triggering membrane binding.     

ATP-dependent interactions between Escherichia coli Min proteins and the phospholipid membrane in vitro

Proper placement of the division apparatus in Escherichia coli requires pole-to-pole oscillation of the MinC division inhibitor. MinC dynamics involves a membrane association-dissociation cycle that is driven by the activities of the MinD ATPase and the MinE topological specificity factor, which themselves undergo coupled oscillatory localization cycles. To understand the biochemical mechanisms underlying Min protein dynamics, we studied the interactions of purified Min proteins with phospholipid vesicles and the role of ATP in these interactions. We show that (i) the ATP-bound form of MinD (MinD.ATP) readily associates with phospholipid vesicles in the presence of Mg(2+), whereas the ADP-bound form (MinD.ADP) does not; (ii) MinD.ATP binds membrane in a self-enhancing fashion; (iii) both MinC and MinE can be recruited to MinD.ATP-decorated vesicles; (iv) MinE stimulates dissociation of MinD.ATP from the membrane in a process requiring hydrolysis of the nucleotide; and (v) MinE stimulates dissociation of MinC from MinD.ATP-membrane complexes, even when ATP hydrolysis is blocked. The results support and extend recent work by Z. Hu et al. (Z. Hu, E. P. Gogol, and J. Lutkenhaus, Proc. Natl. Acad. Sci. USA 99:6761-6766, 2002) and support models of protein oscillation wherein MinE induces Min protein dynamics by stimulating the conversion of the membrane-bound form of MinD (MinD.ATP) to the cytoplasmic form (MinD.ADP). The results also indicate that MinE-stimulated dissociation of MinC from the MinC-MinD.ATP-membrane complex can, and may, occur prior to hydrolysis of the nucleotide.     

Membrane binding by MinD involves insertion of hydrophobic residues within the C-terminal amphipathic helix into the bilayer

MinD binds to phospholipid vesicles in the presence of ATP and is released by MinE, which stimulates the MinD ATPase. Membrane binding requires a short conserved C-terminal region, which has the potential to form an amphipathic helix. This finding has led to a model in which the binding of ATP regulates the formation or accessibility of this helix, which then embeds in the membrane bilayer. To test this model, we replaced each of the four hydrophobic residues within this potential helix with tryptophan or a charged residue. Introduction of a negatively charged amino acid decreased membrane binding of MinD and its ability to activate MinC. In contrast, mutants with tryptophan substitutions retained the ability to bind to the membrane and activate MinC. Fluorescence emission spectroscopy analysis of the tryptophan mutants F263W, L264W, and L267W confirmed that these tryptophan residues did insert into the hydrophobic interior of the bilayer. We conclude that membrane binding by MinD involves penetration of the hydrophobic residues within the C-terminal amphipathic helix into the hydrophobic interior of the bilayer.     

Differences in MinC/MinD sensitivity between polar and internal Z rings in Escherichia coli

In Escherichia coli the Z ring has the potential to assemble anywhere along the cell length but is restricted to midcell by the action of negative regulatory systems, including Min. In the current model for the Min system, the MinC/MinD division inhibitory complex is evenly distributed on the membrane and can disrupt Z rings anywhere in the cell; however, MinE spatially regulates MinC/MinD by restricting it to the cell poles, thus allowing Z ring formation at midcell. This model assumes that Z rings formed at different cellular locations have equal sensitivity to MinC/MinD in the absence of MinE. However, here we report evidence that differences in MinC/MinD sensitivity between polar and nonpolar Z rings exists even when there is no MinE. MinC/MinD at proper levels is able to block minicell production in Δmin strains without increasing the cell length, indicating that polar Z rings are preferentially blocked. In the FtsZ-I374V strain (which is resistant to MinC(C)/MinD), wild-type morphology can be easily achieved with MinC/MinD in the absence of MinE. We also show that MinC/MinD at proper levels can rescue the lethal phenotype of a min slmA double deletion mutant, which we think is due to the elimination of polar Z rings (or FtsZ structures), which frees up FtsZ molecules for assembly of Z rings at internal sites to rescue division and growth. Taken together, these data indicate that polar Z rings are more susceptible to MinC/MinD than internal Z rings, even when MinE is absent.     

Positioning of the MinE binding site on the MinD surface suggests a plausible mechanism for activation of the Escherichia coli MinD ATPase during division site selection

Division site selection in Escherichia coli requires that the MinD protein interact with itself and with MinC and MinE. MinD is a member of the NifH-ArsA-Par-MinD subgroup of ATPases. The MinE-MinD interaction results in activation of MinD ATPase activity in the presence of membrane vesicles. The sites within MinD responsible for its interaction with MinC and MinE were studied by site-directed mutagenesis and yeast two-hybrid analysis, guided by the known three-dimensional structure of MinD proteins. This provided evidence that MinC and MinE bind to overlapping sites on the MinD surface. The results also suggested that MinE and the invariant Lys11 residue in the ATPase P-loop of MinD compete for binding to a common site within the MinD structure, thereby providing a plausible structural basis for the ability of MinE to activate the ATPase activity of MinD.     

Recruitment of MinC,an inhibitor of Z-ring formation,to the membrane in Escherichia coli: role of MinD and MinE

In Escherichia coli, the min system prevents division away from midcell through topological regulation of MinC, an inhibitor of Z-ring formation. The topological regulation involves oscillation of MinC between the poles of the cell under the direction of the MinDE oscillator. Since the mechanism of MinC involvement in the oscillation is unknown, we investigated the interaction of MinC with the other Min proteins. We observed that MinD dimerized in the presence of ATP and interacted with MinC. In the presence of a phospholipid bilayer, MinD bound to the bilayer and recruited MinC in an ATP-dependent manner. Addition of MinE to the MinCD-bilayer complex resulted in release of both MinC and MinD. The release of MinC did not require ATP hydrolysis, indicating that MinE could displace MinC from the MinD-bilayer complex. In contrast, MinC was unable to displace MinE bound to the MinD-bilayer complex. These results suggest that MinE induces a conformational change in MinD bound to the bilayer that results in the release of MinC. Also, it is argued that binding of MinD to the membrane activates MinC.     

The conserved C-terminal tail of FtsZ is required for the septal localization and division inhibitory activity of MinCC/MinD

The Escherichia coli Min system contributes to spatial regulation of cytokinesis by preventing assembly of the Z ring away from midcell. MinC is a cell division inhibitor whose activity is spatially regulated by MinD and MinE. MinC has two functional domains of similar size, both of which have division inhibitory activity in the proper context. However, the molecular mechanism of the inhibitory action of either domain is not very clear. Here, we report that the septal localization and division inhibitory activity of MinC^C/MinD requires the conserved C-terminal tail of FtsZ. This tail also mediates interaction with two essential division proteins, ZipA and FtsA, to link FtsZ polymers to the membrane. Overproduction of MinC^C/MinD displaces FtsA from the Z ring and eventually disrupts the Z ring, probably because it also displaces ZipA. These results support a model for the division inhibitory action of MinC/MinD. MinC/MinD binds to ZipA and FtsA decorated FtsZ polymers located at the membrane through the MinC^C/MinD–FtsZ interaction. This binding displaces FtsA and/or ZipA, and more importantly, positions MinC^N near the FtsZ polymers making it a more effective inhibitor.     

Topological regulation of cell division in Escherichia coli involves rapid pole to pole oscillation of the division inhibitor MinC under the control of MinD and MinE

Placement of the Z ring at midcell in Escherichia coli is assured by the action of the min system, which blocks usage of potential division sites that exist at the cell poles. This activity of min is achieved through the action of an inhibitor of division, MinC, that is activated by MinD and topologically regulated by MinE. In this study, we have used a functional GFP-MinC fusion to monitor the location of MinC. We find that GFP-MinC is a cytoplasmic protein in the absence of the other Min proteins. The addition of MinD, a peripheral membrane protein that interacts with MinC, results in GFP-MinC appearing on the membrane. In the presence of both MinD and MinE, GFP-MinC oscillates rapidly between the halves of the cell. Thus, MinC is positioned by the other Min products, but in a dynamic manner so that it is in position to inhibit Z ring assembly away from midcell.     

MinD and role of the deviant Walker A motif,dimerization and membrane binding in oscillation

The ATPase activity of MinD is required for it to oscillate between the ends of the cell and spatially regulate cell division in Escherichia coli. It is a member of a functionally diverse subgroup of ATPases which are involved in activities ranging from nitrogen fixation (NifH) to plasmid segregation (ParA). All members of the subgroup have a deviant Walker A motif which contains a conserved 'signature' lysine that characterizes this subgroup. In the NifH homodimer the signature lysines make intermonomer contact with the bound nucleotides indicating a role in ATP hydrolysis. ATP binding to NifH leads to formation of an active dimer that associates with a partner that is also a dimer. Because ATP hydrolysis is coupled to formation of the complex, the complex is only transient. In the presence of ATP MinD binds MinC and goes to the membrane, however, the ATPase is not stimulated and the complex is stable. Subsequent interaction of this complex with MinE, however, leads to ATPase stimulation and release of the Min proteins from the membrane. The sequential interaction of MinD with these two proteins, which is dictated by the membrane, is critical to the oscillatory mechanism involved in spatial regulation of division.     

Topological regulation of cell division in E. coli. spatiotemporal oscillation of MinD requires stimulation of its ATPase by MinE and phospholipid.

Topological regulation of cell division in E. coli requires positioning a cell division inhibitor, MinC, at the poles of the cell, thus restricting the potential for division to midcell. This positioning is achieved through a rapid oscillation of MinC from pole to pole, a process requiring MinD and MinE. However, the mechanistic basis for this oscillation is not known. Here we report that MinE stimulates MinD ATPase activity, but only in the presence of phospholipid vesicles. Analysis of MinE mutants demonstrates that this stimulation is required for MinD oscillation and suggests that the level of stimulation determines the period of the oscillation. A model is presented in which the requirements for the MinD ATPase contribute spatial and temporal inputs that provide the mechanistic basis for the oscillation.     

Bacterial cell division: a moving MinE sweeper boggles the MinD

Placement of the division site in Escherichia coli is determined in part by three Min proteins. Recent studies have shown that MinE, previously thought to form a static ring near the division site at the midcell position, actually joins MinC and MinD in their rapid oscillation between the cell poles.     

The MinD protein is a membrane ATPase required for the correct placement of the Escherichia coli division site.

The proper placement of the cell division site in Escherichia coli requires the site-specific inactivation of potential division sites at the cell poles in a process that is mediated by the MinC, MinD and MinE proteins. During the normal division cycle MinD plays two roles. It activates the MinC-dependent mechanism that is responsible for the inactivation of potential division sites and it also renders the division inhibition system sensitive to the topological specificity factor MinE. MinE suppresses the division block at the normal division site at mid-cell but not all cell poles, thereby ensuring the normal division pattern. In this study the MinD protein was purified to homogeneity and shown to bind ATP and to have ATPase activity. When the putative ATP binding domain of MinD was altered by site-directed mutagenesis, the mutant protein was no longer able to activate the MinC-dependent division inhibition system. Immunoelectron microscopy showed that MinD was located in the inner membrane region of the cell envelope. These results show that MinD is a membrane ATPase and suggest that the ATPase activity plays an essential role in the functions of the MinD protein during the normal division process.     

The switch I and II regions of MinD are required for binding and activating MinC

MinD and MinC cooperate to form an efficient inhibitor of Z-ring formation that is spatially regulated by MinE. MinD activates MinC by recruiting it to the membrane and targeting it to a septal component. To better understand this activation, we have isolated loss-of-function mutations in minD and carried out site-directed mutagenesis. Many of these mutations block MinC-MinD interaction; however, they also prevent MinD self-interaction and membrane binding, suggesting that they affect nucleotide interaction or protein folding. Two mutations in the switch I region (MinD box) and one mutation in the switch II region had little affect on most MinD functions, such as MinD self-interaction, membrane binding, and MinE stimulation; however, they did eliminate MinD-MinC interaction. Two additional mutations in the switch II region did not affect MinC binding. Further study revealed that one of these allowed the MinCD complex to target to the septum but was still deficient in blocking division. These results indicate that the switch I and II regions of MinD are required for interaction with MinC but not MinE and that the switch II region has a role in activating MinC.     

Dynamic localization cycle of the cell division regulator MinE in Escherichia coli

The MinC protein directs placement of the division septum to the middle of Escherichia coli cells by blocking assembly of the division apparatus at other sites. MinD and MinE regulate MinC activity by modulating its cellular location in a unique fashion. MinD recruits MinC to the membrane, and MinE induces MinC/MinD to oscillate rapidly between the membrane of opposite cell halves. Using fixed cells, we previously found that a MinE-green fluorescent protein fusion accumulated in an annular structure at or near the midcell, as well as along the membrane on only one side of the ring. Here we show that in living cells, MinE undergoes a rapid localization cycle that appears coupled to MinD oscillation. The results show that MinE is not a fixed marker for septal ring assembly. Rather, they support a model in which MinE stimulates the removal of MinD from the membrane in a wave-like fashion. These waves run from a midcell position towards the poles in an alternating sequence such that the time-averaged concentration of division inhibitor is lowest at midcell.     

ZipA is required for targeting of DMinC/DicB, but not DMinC/MinD, complexes to septal ring assemblies in Escherichia coli

The MinC division inhibitor is required for accurate placement of the septal ring at the middle of the Escherichia coli cell. The N-terminal domain of MinC ((Z)MinC) interferes with FtsZ assembly, while the C-terminal domain ((D)MinC) mediates both dimerization and complex formation with either MinD or DicB. Binding to either of these activators greatly enhances the division-inhibitory activity of MinC in the cell. The MinD ATPase plays a crucial role in the rapid pole-to-pole oscillation of MinC that is proposed to force FtsZ ring formation to midcell. DicB is encoded by one of the cryptic prophages on the E. coli chromosome (Qin) and is normally not synthesized. Binding of MinD or DicB to (D)MinC produces complexes that have high affinities for one or more septal ring-associated targets. Here we show that the FtsZ-binding protein ZipA is required for both recruitment of the (D)MinC/DicB complex to FtsZ rings and the DicB-inducible division block normally seen in MinC(+) cells. In contrast, none of the known FtsZ-associated factors, including ZipA, FtsA, and ZapA, appear to be specifically required for targeting of the (D)MinC/MinD complex to rings, implying that the two MinC/activator complexes must recognize distinct features of FtsZ assemblies. MinD-dependent targeting of MinC may occur in two steps of increasing topological specificity: (i) recruitment of MinC from the cytoplasm to the membrane, and (ii) specific targeting of the MinC/MinD complex to nascent septal ring assemblies on the membrane. Using membrane-tethered derivatives of MinC, we obtained evidence that both of these steps contribute to the efficiency of MinC/MinD-mediated division inhibition.     

Structural and functional studies of MinD ATPase: implications for the molecular recognition of the bacterial cell division apparatus

Proper placement of the bacterial cell division site requires the site-specific inactivation of other potential division sites. In Escherichia coli, selection of the correct mid-cell site is mediated by the MinC, MinD and MinE proteins. To clarify the functional role of the bacterial cell division inhibitor MinD, which is a membrane-associated ATPase that works as an activator of MinC, we determined the crystal structure of a Pyrococcus furiosus MinD homologue complexed with a substrate analogue, AMPPCP, and with the product ADP at resolutions of 2.7 and 2.0 A, respectively. The structure reveals general similarities to the nitrogenase iron protein, the H-Ras p21 and the RecA-like ATPase domain. Alanine scanning mutational analyses of E.coli MinD were also performed in vivo. The results suggest that the residues around the ATP-binding site are required for the direct interaction with MinC, and that ATP binding and hydrolysis play a role as a molecular switch to control the mechanisms of MinCDE-dependent bacterial cell division.     

MinD and MinE Interact with Anionic Phospholipids and Regulate Division Plane Formation in Escherichia coli

The Min proteins (MinC, MinD, and MinE) form a pole-to-pole oscillator that controls the spatial assembly of the division machinery in Escherichia coli cells. Previous studies identified that interactions of MinD with phospholipids positioned the Min machinery at the membrane. We extend these studies by measuring the affinity, kinetics, and ATPase activity of E. coli MinD, MinE, and MinDE binding to supported lipid bilayers containing varying compositions of anionic phospholipids. Using quartz crystal microbalance measurements, we found that the binding affinity (K_d) for the interaction of recombinant E. coli MinD and MinE with lipid bilayers increased with increasing concentration of the anionic phospholipids phosphatidylglycerol and cardiolipin. The K_d for MinD (1.8 μm) in the presence of ATP was smaller than for MinE (12.1 μm) binding to membranes consisting of 95:5 phosphatidylcholine/cardiolipin. The simultaneous binding of MinD and MinE to membranes revealed that increasing the concentration of anionic phospholipid stimulates the initial rate of adsorption (k_on). The ATPase activity of MinD decreased in the presence of anionic phospholipids. These results indicate that anionic lipids, which are concentrated at the poles, increase the retention of MinD and MinE and explain its dwell time at this region of bacterial cells. These studies provide insight into interactions between MinD and MinE and between these proteins and membranes that are relevant to understanding the process of bacterial cell division, in which the interaction of proteins and membranes is essential.     

The three-dimensional structure of septum site-determining protein MinD from Pyrococcus horikoshii OT3 in complex with Mg-ADP

BACKGROUND: In Escherichia coli, the cell division site is determined by the cooperative activity of min operon products MinC, MinD, and MinE. MinC is a nonspecific inhibitor of the septum protein FtsZ, and MinE is the supressor of MinC. MinD plays a multifunctional role. It is a membrane-associated ATPase and is a septum site-determining factor through the activation and regulation of MinC and MinE. MinD is also known to undergo a rapid pole-to-pole oscillation movement in vivo as observed by fluorescent microscopy. RESULTS: The three-dimensional structure of the MinD-2 from Pyrococcus horikoshii OT3 (PH0612) has been determined at 2.3 A resolution by X-ray crystallography using the Se-Met MAD method. The molecule consists of a beta sheet with 7 parallel and 1 antiparallel strands and 11 peripheral alpha helices. It contains the classical mononucleotide binding loop with bound ADP and magnesium ion, which is consistent with the suggested ATPase activity. CONCLUSIONS: Structure analysis shows that MinD is most similar to nitrogenase iron protein, which is a member of the P loop-containing nucleotide triphosphate hydrolase superfamily of proteins. Unlike nitrogenase or other member proteins that normally work as a dimer, MinD was present as a monomer in the crystal. Both the 31P NMR and Malachite Green method exhibited relatively low levels of ATPase activity. These facts suggest that MinD may work as a molecular switch in the multiprotein complex in bacterial cell division.